Biodegradable and other naturally degradable polymers have been attracting attention from the viewpoint of environmental protection. Researchers are therefore conducting active research on poly(lactic acid)s, copolymers of lactic acid and other aliphatic hydroxycarboxylic acids, polyesters of aliphatic polyhydric alcohols with aliphatic polycarboxylic acids, and the like.
In particular, poly(lactic acid)s are preferable materials since they are produced from agricultural products and thus free from the problem of resource depletion. In recent years, L-lactic acid used as the raw material can be produced by fermentation in large quantities and at low cost. However, poly(lactic acid)s have the drawback of being rigid, brittle and inflexible because of their high crystallinity and rigid molecular structure. Therefore, poly(lactic acid)s per se are not suitable for applications that require flexibility, such as films and packaging.
Poly(lactic acid)s can be softened, for example, by adding plasticizers, blending soft polymers, or carrying out copolymerization. However, when blending soft polymers, usable soft polymers are limited to biodegradable resins such as polybutylene succinate from the viewpoint of biodegradability. Such biodegradable resins have to be added in large quantities to impart sufficient flexibility, and the addition in large quantities may impair this characteristic of poly(lactic acid)s. Copolymerization changes the physical properties such as melting point and heat resistance, owing to the decrease in crystallinity and glass transition temperature.
Further, when plasticizers are added, they are likely to migrate (bleed) to the surface, thereby staining the surface or impairing the transparency of the molded article. To solve this problem, various plasticizers as mentioned below have been proposed.
For example, Japanese Unexamined Patent Publication No. 1992-335060 discloses a composition comprising a poly(lactic acid) and a plasticizer, and shows diisopropyl adipate and dioctyl sebacate as effective plasticizers. However, their plasticizing effects are small and the composition is too inflexible to be used for general-purpose films. This publication also discloses diethyl phthalate as a phthalic acid plasticizer. However, diethyl phthalate has a low plasticizing effect and is likely to bleed out and volatilize.
Japanese Unexamined Patent Publication Nos. 1999-35808 and 1999-181262 disclose use of an ether ester plasticizer in a lactic acid polymer comprising a poly(lactic acid) as the main component. However, the ether ester plasticizer itself has insufficient water resistance, and when it is used in films or like molded articles, the molded articles have drawbacks such as low tensile strength.
Japanese Unexamined Patent Publication No. 2000-136300 discloses a lactic acid polymer comprising polypropylene glycol dibenzoate as a plasticizer. However, because of its poor compatibility with resins, this plasticizer has a low plasticizing effect and is difficult to use in general-purpose films.
Japanese Unexamined Patent Publication No. 2000-198908 discloses use of acetyl tributyl citrate, dioctyl phthalate or diisodecyl adipate singly as a plasticizer in a poly(lactic acid). However, these plasticizers easily bleed out and have a poor plasticizing effect.
Japanese Unexamined Patent Publication No. 2001-294736 discloses use of bis(butyldiglycol) adipate or the like singly as a plasticizer in a poly(lactic acid) as an aliphatic polyester resin. However, these plasticizers easily bleed out and have poor water resistance and low plasticizing effect.
Thus, no satisfactory plasticizers for biodegradable resins have been developed yet. One idea may be to use two or more plasticizers to compensate for each other's shortcomings. However, such use makes the production process complicated, and some plasticizers are difficult to mix uniformly because of their differences in compatibility with the base resin.